Models, Observations Not So Far Apart On Planet’s Response To Greenhouse Gas Emissions

(Click to enlarge) The Earth may be in an “adjusting” period to greenhouse gases. (Credit: Pibwl/ Wikimedia Commons)

How hot our planet will become for a given amount of greenhouse gases is a key number in climate change. As the calculation of how much warming is locked in by a given amount of emissions, it is crucial for global policies to curb global warming.

(From Science Daily) — It is also one of the most hotly debated numbers in climate science. Observations in the past decade seem to suggest a value that is lower than predicted by models. But a University of Washington study shows that two leading methods for calculating how hot the planet will get are not as far apart as they have appeared.

In climate science, the climate sensitivity is how much the surface air temperature will increase if you double carbon dioxide from pre-Industrial levels and then wait a very long time for Earth’s temperature to fully adjust. Recent observations predicted that the climate sensitivity might be less than that suggested by models.

The new study, published April 17 in Nature Climate Change, focuses on the lag time in Earth’s response. According to most models of climate change, during the early stages of global warming the sensitivity to greenhouse gas emissions is relatively small. As the ocean catches up and feedbacks kick in, however, the sensitivity increases and the warming rate speeds up. The new study shows that when this difference is factored in, the observations and climate models are in agreement, with recent observations supporting a previously accepted long-term climate sensitivity of about 2.9 degrees Celsius.

“The key is that you have to compare the models to the observations in a consistent way,” said author Kyle Armour, a UW assistant professor of oceanography and atmospheric sciences. “This apples-to-apples approach — where you factor in how long the planet has been adjusting to a change in its atmosphere — shows that climate sensitivity in the models is actually in line with what has been seen in the recent observations.”

The planet’s temperature takes thousands of years to fully adjust to a shift in the makeup of its atmosphere — the warming Earth has experienced to date is just a taste of what is in store. Early climate studies suggested that if the amount of carbon dioxide in the atmosphere doubled from pre-Industrial levels (we’re now about 1.4 times) the planet would eventually warm by about 3 degrees C, with possible values as high as 5 or 6 degrees C.

But recent observations of warming so far and emissions to date have suggested that climate sensitivity may be just under 2 degrees Celsius, with a maximum possible value of 4 degrees C.